1. Technical Field
This disclosure relates generally to switching regulators, control circuits thereof, and methods of controlling operations thereof, and more particularly to a switching regulator configured to switch between PWM control and PFM or VFM control in accordance with load conditions, a control circuit of the switching regulator, and a method of controlling the operation of the switching regulator.
2. Description of the Related Art
In these years, environmental concerns have required electronic apparatuses to reduce power consumption. This tendency is particularly notable in battery-driven electronic apparatuses. Generally speaking, for power savings, it is important to reduce power consumed in electronic apparatuses and to reduce unnecessary power consumption by improving the efficiency of power supply circuits themselves.
Non-isolated switching regulators using an inductor are popular as high-efficiency power supply circuits for small-size electronic apparatuses. Roughly speaking, three methods are known as control methods of switching regulators.
The first one is PWM (Pulse Width Modulation) control that keeps output voltage constant by varying the duty cycle of clock pulses of a fixed frequency. The second one is PFM (Pulse Frequency Modulation) control that keeps output voltage constant by varying a clock cycle with a fixed pulse width. The third one is VFM (Variable Frequency Modulation) control that keeps output voltage constant by controlling clock output of a fixed pulse width in accordance with output voltage error.
There are two types of PFM control: one is to vary frequency in a stepless manner and the other is to vary frequency in a pseudo manner by thinning out a clock signal of a frequency used in PFM control.
In PWM control, on-off control of a switching transistor is performed at a constant frequency even with a light load. Accordingly, PWM control suffers from degraded efficiency at a light-load time when the current output to a load is small. Meanwhile, in PFM control or VFM control, the frequency of a signal to cause switching of the switching transistor varies in accordance with a connected load. Accordingly, PFM control or VFM control is more efficient for a light load than PWM control although there is a considerable effect of noise or ripple on devices.
For the foregoing reasons, conventionally, improvement in power supply efficiency is achieved for a light load through a heavy load by switching between PWM control and PFM control or switching between PWM control and VFM control in accordance with load conditions.
In general, the load condition is detected by inserting a resistor for output current detection between a supply voltage forming an input voltage and an output terminal and detecting an output current from the output terminal. According to this method, however, power loss due to the resistor for output current detection increases as the output current increases. Accordingly, this method is not suitable for small electronic devices using a battery as a power supply. Therefore, a method that indirectly detects a load condition using the voltage level of an error amplifier circuit is proposed as a method that does not use a resistor for output current detection. (See, for example, Patent Document described below.)
[Patent Document] Japanese Patent No. 3647811
However, an integrating circuit for eliminating the effect of a ripple component superposed on output voltage is attached to the error amplifier circuit.
Usually, the integrating circuit is added to the error amplifier circuit as a phase compensator circuit, and is optimized for the operating frequency used at the time of PWM control. Therefore, when the operating frequency becomes lower than at the time of PWM control (or the pulses of a pulse signal for PWM control are thinned out) as in the case of PFM control, the output of the integrating circuit, which is a differential error output, effectively functions immediately after a switching operation, but the output voltage of the error amplifier circuit becomes 0 V or equal to a supply voltage level and does not effectively function as a load current detection signal when the switching operation is stopped because of, for example, thinning out the pulses of a pulse signal for PWM control.
Accordingly, at the time of PFM control, the output voltage of the error amplifier circuit cannot be kept constant with respect to a load current, so that the relationship between the output voltage of the error amplifier circuit and the load current becomes variable. This causes a problem in that the load current at the switching time of control methods cannot be determined with accuracy compared with the method that measures the load current using a resistor for output current detection.